The invention relates to a process of producing crystals in crystallographic point group 32 which correspond to the comprehensive chemical formula APO.sub.4, symbol A being a chemical element taken from the group Al and Ga, P is phosphorus and O is oxygen from heated solutions of salts of group A in phosphorus acid. The salts may also be produced by heating the corresponding oxide in the phosphorus acid under reflux.
The hitherto known processes of the kind specified for growing monocrystals of the type specified require the use of sealed pressure vessels because of the utilized process parameters such as pressures, temperatures and acid concentrations. This makes it difficult to visually monitor the growth of the crystals. In addition, such sealed pressure vessels are costly and always present certain safety risks.
By way of example a process of producing AlPO.sub.4 -monocrystals is known in which total pressures of between 7 and 35 bar are applied inside the pressure vessel containing the heated solution. Also known in this context is the use of 6.1 molar H.sub.3 PO.sub.4 at 250.degree. C. or 70 to 700 bar.